A pre-mixed charge compression ignition combustion engine has been known, in which air-fuel mixture gas including air and fuel is formed in a combustion chamber and the air-fuel mixture is self-ignited to be combusted (burned) by compressing the air-fuel mixture during a compression stroke. In the pre-mixed charge compression ignition engine, an air-fuel ratio (a ratio of air to fuel) can be extremely large (lean) and a high compression ratio can be adopted. Therefore, fuel consumption may be improved and an amount of NOx may be decreased, if the engine is operated (or driven) by pre-mixed charge compression ignition combustion in a wider driving area.
In the self-ignition combustion, the compressed air-fuel mixture is self-ignited substantially simultaneously at multiple ignition points and the combustion takes place (or lasts) in an extremely short period. This causes noise to be large, especially under a high load driving condition where an amount of fuel is large, because a pressure in the combustion chamber (or a chamber pressure) increases rapidly. The reason why the pre-mixed charge compression ignition combustion can not be used under the high load driving condition is that such noise becomes excessively large.
Meanwhile, if the self-ignition combustion can be made to proceed moderately (relatively slowly), it is possible to reduce such combustion noise since a rising rate (or an increasing ratio) in the chamber pressure decreases. With this view, in a conventional pre-mixed charge compression ignition engine, an area (where EGR gas layer and air layer have in contact with each other) where a temperature gradient is large is formed in the combustion chamber by introducing through one of two intake ports high temperature gas (or the EGR gas) which has been displaced from the combustion chamber and by introducing through the other intake ports low temperature air during an intake stroke, and then fuel is injected into the area. It is inferred that this enables the self-ignition combustion to proceed from the higher temperature area to the lower temperature area according to the temperature gradient, and therefore, suppressing the rapid combustion can be achieved (see Japanese Patent Application Laid-Open (kokai) No. 2001-214741, claim 1, paragraphs 0028-0029, 0044-0049, FIGS. 4, 5, and 26(a)).
However, according to various examinations, the inventors have found that the temperature gradient (or spatial temperature un-uniformity (“un-unifromity” means “inhomogeneity” in this application) of the air-fuel mixture) which has been formed in the combustion chamber prior to the compression stroke decreases (or substantially disappears) during an early part of the compression stroke. Thus, in the conventional pre-mixed charge compression ignition combustion engine, the appropriate temperature un-uniformity of the mixture gas in the combustion chamber can not exist when a reaction relating to the self-ignition starts in the vicinity of a top dead center at the end of the compression stroke. As a result, there is a problem that it is not possible to moderate the combustion appropriately.